1. Field of the Invention
The present invention relates to multi-layer ceramic electronic parts having, for example, a laminated body of internal electrode patterns and ceramic layers therein, at end portions of which are provided external electrodes so as to conduct to the internal electrodes, and in particular to multi-layer ceramic electronic parts in which a material common with the ceramic material for forming the ceramic layers of the laminated body is added into at least a portion of the external electrodes thereof.
2. Description of Related Art
As electronic components of a laminated type can be listed, for example, a laminated capacitor, a laminated inductor, a laminated piezo element, a laminated filter, a ceramic multi-layer circuit board, etc.
For example, in the laminated ceramic capacitor, as being the most representative one of the laminated electronic components, a large number of layers are piled up or laminated, each having an internal electrode and made of dielectric material, wherein the above-mentioned internal electrodes are pulled out one another to the end surfaces of the laminated body opposing to each other. On the end surfaces to which those internal electrodes are pulled out, external electrodes are formed, and those external electrodes are also connected to the above-mentioned internal electrodes, respectively.
The above-mentioned laminated body 3 of the laminated ceramic capacitor has a layer construction as shown in FIG. 3, for example. Namely, the ceramic layers 7,7 . . . , each having the internal electrode 5 or 6 and made of dielectric material, are laminated in an order as shown in FIG. 3, and further, on both sides (i.e., on upper and lower sides) thereof are piled up or laminated the ceramic layers 7,7 . . . in a plurality thereof, on which no such the electrode 5 or 6 is formed, respectively. Upon the end portions of the laminated body 3 having such a layer structure therein, the internal electrodes 5 and 6 expose one another, and as shown in FIG. 1, the above-mentioned external electrodes 2 and 2 are formed at the end portions of this laminated body 3.
Such a laminated ceramic capacitor, ordinarily, is not manufactured one by one, in or as a unit of one part as shown in FIG. 3, but actually is obtained by a manufacturing method which will be described below. Namely, first of all, minute ceramic powder and organic binder are mixed to prepare a slurry, and it is extended thinly on a carrier film made from a polyethylene terephthalate film, etc., by the doctor blade method. Then, it is dried to be formed into a ceramic green sheet. Next, this ceramic green sheet is cut out into a desired size by a cutting head, while being mounted on the supporting sheet, and is printed with a conductive paste on one side surface thereof by a screen printing method, and is dried. With this, the ceramic green sheets 1a and 1b are obtained, on each of which plural sets of the internal electrode patterns 2a and 2b are aligned or arranged in the vertical and horizontal directions, as shown in FIG. 4.
Next, plural pieces of the ceramic green sheets 1a and 1b, each having the above-mentioned internal electrode patterns 2a or 2b thereon, are piled up or laminated, and further are piled several pieces of the ceramic green sheets 1,1 . . . not having the internal electrodes 2a or 2b, at the top and the bottom thereof. They are compressed and put together, thereby forming the laminated body. Here, the above ceramic green sheets 1a and 1b are piled up on one another, on which the internal electrode patterns 2a and 2b are shifted by half a length in a longitudinal direction thereof. After that, the laminated body is cut out into a desired size, thereby manufacturing the laminated raw chips, and those raw chips are baked. In this manner are obtained the laminated bodies as shown in FIG. 3.
Next, this baked laminated body 3 is applied with a conductor paste on both ends thereof and is baked, and on the surface of the baked conductive film is treated a plating, thereby completing the laminated ceramic capacitor formed with the external electrodes at both ends thereof, as shown in FIG. 1.
The multi-layer ceramic electronic parts, such as the laminated ceramic capacitor mentioned above, is mounted on a circuit board and is soldered at the external electrodes 2 and 2 on both ends thereof on land electrodes of the circuit board.
However, such a laminated ceramic capacitor generates thermal stress within the laminated body 3 due to heat-shock when being soldered at the external electrodes thereof or due to a change of a circumference temperature under the condition of use after the soldering. With this thermal stress, in particular in end portions of the external electrodes 2 and 2 of the laminated body 3, cracks can easily occur. The cracks occurring in the laminated body 3 bring about a lowering in insulation due to the invasion of moisture inside and a lowering in static capacitance due to discontinuity of the internal electrodes 5 and 6, thereby causing a low reliability thereof.
Such a thermal stress causing the cracks in the laminated body occurs due to the difference in thermal expansion ratio between the ceramic material, which is a main ingredient for forming the laminated body 3, and the conductor, which is a main ingredient for forming the external electrodes 2 and 2. Then, conventionally, a measure was taken, by adding into the conductor paste for forming the external electrodes 2 and 2 the ceramic material for forming the ceramic layer 7 as a common material, thereby minimizing the difference between the ceramic layer 7 and the external electrodes 2 and 2 in physical properties, such as the thermal stress therein.
However, if a large amount of the common material, i.e., the ceramic material for forming the ceramic layer 7, is put or added into the conductor paste for forming the external electrodes 2 and 2, stickiness or adhesiveness of the external electrodes 2 and 2 onto the external electrodes 5 and 6 comes to be inferior, i.e., the connecting resistance therebetween becomes large and also the electrical properties thereof deteriorate. Further the stickiness or adhesiveness onto the solder or Sn plating also deteriorates. As a result of this, the solder wetability of the external electrodes 2 and 2 becomes inferior, therefore mis-mounting easily occurs when mounting the multi-layer ceramic electronic parts on the circuit board.
An object, according to the present invention, for solving the problems in the conventional art mentioned above, is to provide a multi-layer ceramic electronic part, wherein the cracks in the laminated body hardly occur due to heat-shock accompanying a change in temperature under the conditions of being soldered and during the use thereafter, and further, the adhesiveness between the external and internal electrodes and the adhesiveness of solder onto the external electrode are superior, as well, thereby also being superior in soldering property with the external electrodes.
According to the present invention, for achieving the above-mentioned object, a first conductor film 21 containing a relatively small amount of a common material with respect to the conductor component is formed only on the end surfaces of the laminated body 3, upon which the edges of the internal electrodes 5 and 6 are led out, and a second conductor film 22 containing no common material or containing a very small amount thereof is provided on this first conductor film 21 to cover a portion of the side surfaces adjacent to the laminated body 3, thereby forming the external electrodes 2 and 2.
Namely, according to the present invention, there is provided a multi-layer ceramic electronic part, comprising:
a laminated body 3 in which a ceramic layer 7 and internal electrodes 5 and 6 are laminated one another; and
external electrodes 2 and 2 are provided at end portions of the laminated body 3, in which the internal electrodes 5 and 6 opposing to each other reach to either one of at least a pair of edges of the ceramic layer 7, thereby leading out the internal electrodes 5 and 6 opposing to each other to either one of end surfaces of the laminated body 3, and connecting the internal electrodes 5 and 6 led out to the end surfaces of the laminated body 3 to the external electrodes 2 and 2, respectively, wherein each of the external electrodes 2 and 2 has a first conductor layer 21 which closely contacts with only the end surface of the laminated body 3 and a second conductor layer 22 which covers from the first conductor layer 21 to a portion of a side surface adjacent to the end surface of the laminated body 3, and the first conductor layer 21 is larger than the second conductor layer 22, in percent content of a common material which is commonly used with the ceramic material for forming the ceramic layer 7.
Here, in the multi-layer ceramic electronic part defined above, the second conductor layer 22 of each of the external electrodes 2 and 2 is higher than the first conductor layer 22 in density of the conductor.
Further, in the multi-layer ceramic electronic part defined above, the first conductor layer 21 and the second conductor layer 22 of each of the external electrodes 2 and 2 are formed as one conductor body by baking them at the same time.
In the multi-layer ceramic electronic part, since the first conductor layer 21 of each of the external electrodes 2 and 2, which are provided on the end surfaces of the laminated body 3, contains a relatively small amount of the common material, the adhesiveness onto the surface of the ceramic layers 7 and the adhesiveness to the internal electrodes 5 and 6 is good or satisfactory at the end surfaces of the laminated body 3. Thereby, the contact resistance between the external electrodes 2 and 2 and the internal electrodes 5 and 6 is small, and the separation or exfoliation of the first conductor layers 21 from the end surfaces of the laminated body 3 hardly occurs.
On the other hand, the second conductor layer 22 of each of the external electrodes 2 and 2, which are soldered onto the land electrodes on the circuit board, since they contain no common material or contain a little bit thereof, the density of the conductor is high therein. Therefore, the adhesiveness onto the solder plating and/or Sn plating is good or satisfactory on the surfaces of the external electrodes 2 and 2. As a result of this, the wetability of the external electrodes 2 and 2 with solder is also good, and the soldering strength is also high when the component is mounted.
Furthermore, the second conductor layer 22 of each of the external electrodes 2 and 2, which turn around on the side surfaces of the laminated body 3 in a portion thereof, is large in conductor density, i.e., is small in the percentage content of the material common to the ceramic material forming the ceramic layers 7, therefore the bonding power is small on the surface of the ceramic layers 7 forming the side surfaces of the laminated body 3. Therefore, shifting occurs easily between the second conductor layers 22 and the side surface portions of the laminated body 3 when the temperature changes, so thermal stresses hardly occur within the laminated body 3. Accordingly, the cracks hardly occur in the laminated body 3.
In the multi-layer ceramic electronic part as defined above, it is preferable that the percentage content of the common material in the first conductor layers 21 of the external electrodes 2 and 2 lies from 3 weight % to 40 weight % with respect to 100 weight % of conductor component. If the percentage content of the common material in the first conductor layers 21 is less than 3 weight % with respect to the 100 weight % of conductor component, the adhesiveness thereof to the ceramic layers 7 becomes bad or unsatisfactory, thereby the first conductor layer 21 can be easily exfoliated or peeled off from the end surface of the laminated body 3. On the other hand, if the percentage content of the common material in the first conductor layers 21 exceeds the 40 weight %, the adhesiveness of the first conductor layers 21 onto the internal electrodes 21 becomes bad, then the contact resistance between the external electrodes 2 and 2 and the internal electrodes 5 and 6 becomes too large.
Though the second conductor layer 21 of the external electrodes 2 and 2 must not contain the common material therein, and even in a case where it contains the common material therein, the percentage content thereof is preferably small, such as being equal to or less than 2 weight % to the 100 weight % of the conductor component. If the percentage content of the common material in the second conductor layers 22 of the external electrodes 2 and 2 exceeds the 2 weight % to the 100 weight % of the conductor component, the adhesiveness thereof onto the ceramic layers 7 becomes too good, then the thermal stress within the laminated body 3 becomes large, when the temperature changes, at the end portions of the external electrodes 2 and 2.
In this manner, in the multi-layer ceramic electronic part according to the present invention, not only does the adhesiveness between the external electrodes 2 and 2 and the internal electrodes 5 and 6 at the end surfaces of the laminated body 3, but also the adhesiveness between the external electrodes 2 and 2 and the ceramic layers 7 becomes satisfactory. Also, the adhesiveness upon the surfaces of the external electrodes 2 and 2 onto the solder and the Sn plating becomes satisfactory. As a result of this, the wetability of the external electrodes 2 and 2 onto the solder becomes superior, and the strength of the soldering becomes high when the component is mounted.
Furthermore, the thermal stress accompanying the change of temperature hardly occurs, and cracks hardly occur within the ceramic layers 7.